In this post, an experiment will be conducted on the effects of MP3 quality in relation to dithering and sample rate conversion which are done in the mastering process. The tools used are the following:

Spectral result of the original source audio

The x-axis is the time in seconds while the y-axis is frequencies. The curve blue line is trending upward (because it’s as sweep tone) indicates the change of frequency content with respect to time. So we can say approximately by looking at the plot that at 4 seconds; the signal frequency of the content is at 10,000Hz. The black background/region indicates the absence of signal frequency content.

Test Flow#1: Direct Encoding

In this test, a high resolution test tone of 24-bit/96KHz is directly feed to the Reaper LAME encoder (File – Batch File/Item converter). In Reaper, these are the options: [sample rate=source, channels=source, re-sample if needed=best, Output format=MP3, mode=maximum bit rate/quality]. If you are new to Reaper, you can add MP3 functionality by reading this guide on Reaper DAW Tutorial. And then go to the “Using MP3 with Reaper” section.

For the above test; this is the spectral result of the MP3:

Test flow #1 spectral response

As you can see, very small artifacts are now present and the background is not anymore pure black. This indicates the presence of slight aliasing distortion (those light hazy blue lines) brought by the MP3 encoder sample rate conversion. It is worth noting that LAME encoder converts a 24-bit/96 KHz sample rate to a 16-bit/48 KHz mp3. If it’s directly converted to a standard 16-bit/44.1KHz MP3 using the same process, this is the result:

More aliasing distortion

And now you have much bigger problems of aliasing distortion; fully obvious and more audible. See those lines crossing the ideal signal which are not present in the original test tone? These are distortions/artifacts that can make your MP3 sound bad.